Posted: 1st July 2022
Rw, DnTw, and Dw are all different indices which describe sound insulation performance – or how much noise will transmit through a wall or floor. Having different indices can be confusing and misunderstanding these terms could mean your building fails the final sound tests. This guide gives an explanation of the terms and what you should look out for when designing the acoustic performance of a partition.
An Rw is the weighted sound reduction index. It is a laboratory measured value to identify the airborne sound insulation performance of a building element. It is used for internal or external walls, ceilings/floors, windows, doors, or any separating element. The higher the Rw value, the better that element performs in reducing sound transmission.
Because it is a laboratory-tested level then the Rw does not take into account any flanking noise paths which would occur on site. This means that an Rw 50dB wall is very unlikely to reduce the noise by 50dB on site.
The Dw is a simple level difference – the noise level in the source room minus the sound level in the receiving room. This is the level of sound transmission that we hear when standing in the receiving room.
Sound transmits between rooms through many different paths. There will be direct sound transmission through the wall or floor between the rooms, but also transmission through junctions with the floor/ceiling and side walls, and through doors or windows. Transmission can also occur via columns, pipes, ducts, and structural steels. Noise transmission through these other routes is termed ‘flanking’. Flanking transmission means that the performance of a wall or floor on site will always be lower than the Rw performance suggests.
There are variations on a level difference, including the DnTw. A DnTw normalises the Dw to account for the reverberation time – or echo – in the receiving room. This allows us to compare measured sound insulation results between different rooms and at different times, irrespective of the amount of echo in the receiving room.
In theory, the DnTw sound insulation performance should remain the same when testing between a pair of rooms when they are unfurnished compared to tests when the rooms have been furnished and carpeted.
Building Regulations Approved Document E specifies minimum performance standards in terms of DnTw + C’tr. The C’tr term is an additional correction which penalises poor performance at low frequencies. Some manufacturers publish laboratory sound insulation performance data in terms of Rw + C’tr.
A wall constructed in a laboratory should achieve the same Rw value each time it’s tested. However, the Dw, or DnTw, performance on site will vary between rooms. Factors which will influence the Dw performance on site include:
What this means is that there is no simple correlation between a Rw and a Dw or DnTw. It’s generally reasonable to assume that a Dw will be around 4dB to 5dB lower than the equivalent Rw for masonry or concrete elements, and around 7dB to 10dB lower than the Rw for lightweight timber or metal constructions, but this might not always be the case.
Using the above rules of thumb are a good starting point at the early stages of a design. However, getting the design of separating walls and floors wrong could cost you £000’s if your sound insulation tests fail, or you start receiving complaints from residents living above your venue.
We have extensive experience in designing the sound insulation performance of buildings. This includes using specialist 3D computer noise modelling. This process will allow us to verify at the design stage the required Rw performance for separating elements to ensure that the Dw or DnTw that is achieved on site meets the required criteria.
We will also provide you with acoustic detailing and design of flanking elements so that you can be sure your sound tests will pass.
To discuss your project in more detail with one of our experienced acoustic consultants, and to find out how we can assist in making sure you achieve the required standard of sound insulation, please call us at your local office number, or use the contact form.